This invention relates to apparatus for and methods of detecting a target object and more particularly to apparatus for and methods of optically detecting a target object on a road from a vehicle by also detecting the deviation of the optical axis of the light emitted for the detection. The invention also relates to such apparatus and methods capable of adjusting the orientation of the optical axis optimally.
It has been known to install a laser radar on a motor vehicle for measuring the distance to a vehicle ahead and to cause an alarm signal outputted if the measured distance becomes less than a specified safety distance, thereby advising the driver to keep a larger distance or to automatically control the speed of the vehicle to increase the separation. FIG. 1 shows an example of such a detector 2 installed in a car (or the user""s xe2x80x9cown carxe2x80x9d) 1. The detector 2 includes a laser radar 3. As a beam of laser light 4 is emitted from the laser radar 3 and reflected by the back surface of a car 5 ahead of and moving in the same direction as the user""s car 1, the reflected light is received by the laser radar 3 and the detector 2 measures the time between the emission of the laser light 4 from the laser radar 3 and its reception after being reflected by the car 5 in front and calculates the distance in between based on this measured time. If it is determined that the distance thus calculated is not a safe distance (within which the own car 1 is thought to be able to stop when the car in front 5 is involved in an accident or stops suddenly), a warning signal may be outputted to increase the distance between the cars or the speed of the own car 1 is controlled so as to maintain a safe distance in between.
Such a detector, however, is not cable of correctly measuring the distance to the car 5 in front if the optical axis of the laser light 4 emitted from the laser radar 3 is displaced. FIG. 2 shows such a situation when the angle of light emission from the laser light 4 is displaced upward, for example, as a result of a body deformation due to a collision or because the general loosening of body parts after years of use. In a situation as shown in FIG. 2, the reflected portion of the emitted laser light 4 cannot be received by the laser radar 3 until the separation between the two cars is as shown by the broken lines in FIG. 2. In other words, there were frequently situations with a prior art detector where the car in front could not be detected until the distance in between was too short to be safe.
Even if the detector 2 is properly installed such that the laser radar transmits its light in the desired direction but if the road is sloped as shown in FIG. 3, however, the detector 2 may not be able to properly detect the car 5 in front, the laser light 4 being reflected by the road surface. In other words, the prior art detector could not dependably warn the driver of a potentially dangerous situation.
In view of these problems, Japanese Patent Publication Tokkai 11-194169 disclosed a detector to be installed on a vehicle including detecting means for detecting the tilting angle of the detector itself with respect to the road surface, as well as selecting one of a plurality of laser radars emitting an optimum laser beam according to the detected tilting angle. The detector includes not only a first laser radar for emitting laser light parallel to the road surface for measuring the distance to the car in front but also a second laser radar and a third laser radar for emitting laser light at specified angles towards the road surface in order to detect the sloping angle of the detector itself with respect to the road surface. The detector calculates the sloping angle of its own optical axis on the basis of measurements by the second and third laser radars and then the direction of the optical axis of the laser light emitted from the first laser radar. On the basis of this detection, one of a plurality of detection beams is selected.
This detector is disadvantage firstly because the second and third laser radars are required in addition to the first laser radar and this makes the detector large as a whole and more expensive. Moreover, this detector treats the angles of the optical axes of the laser light as standard values, and the angle of the optical axis of the laser light with respect to the road surface is calculated only indirectly from the angle of the detector with respect to the road surface. Thus, if the angles of the optical axes of the laser light with respect to the detector are different from values which were specified by the design, the angles of the optical axes with respect to the road surface cannot be calculated accurately. Still another disadvantage of this detector is that it can detect the changes in the road condition directly below the detector but the changes in the slope in front of the vehicle cannot be detected.
It is therefore an object of this invention to provide a detector of a simple structure for a vehicle capable of detecting the displacement of the optical axis of the laser light.
It is another object of the invention to provide such a detector capable of using the laser light from its laser radar to detect and correct the displacement of its optical axis and adjusting the angle of its light emission.
A detector for an object on a road surface according to this invention may be characterized as comprising beam emitting means for emitting a beam, beam receiving means for receiving reflected beam of the beam emitted from the beam emitting means by an object, distance measuring means for measuring a distance to the object from the reflected beam received by the beam receiving means, recognizing means for recognizing an overhead beam reflector with a known height based on the reflected beam received by the beam receiving means, distance identifying means for identifying a distance to the overhead beam reflector immediately before it becomes unrecognizable by the recognizing means, and displacement detecting means for detecting a directional displacement of the beam emitted from the beam emitting means based on the distance to the overhead beam identified by the distance identified means. The beam emitting and receiving means may be a laser radar emitting laser light and receiving light reflected from the road surface ahead or an object on the road surface. The distance measuring, recognizing and distance identifying means may be realized by what is referred to as a recognizer unit below, continuously calculating the distance between the device and an overhead beam reflector until it becomes unrecognizable and identifying the calculated distance immediately before it becomes unrecognizable. The displacement detecting means serves to calculate the vertical displacement in the direction of the beam outputted from the laser radar on the basis of the known height of the overhead beam reflector and the height of the detector itself, say, set on a motor vehicle.
The detector may also include correcting means for correcting the directional displacement of the optical axis of the emitted beam detected by the displacement detecting means by calculating a correction angle with an angle calculator and correcting the angle with an angle controller based on the correction angle calculated by the angle calculator. The correcting means may adjust the direction of emission from a laser radar by continuously changing the angle of emission in a vertical direction. Alternatively, a plurality of emission angles may be preselected in a vertical direction and the correcting means may correct the directional displacement by selecting one of these preselected angles.
The detector may further include displacement judging means for judging whether or not the directional displacement detected by the displacement detecting means is within a specified range, and warning means for outputting a warning signal when the displacement judging means judges that the directional displacement detected by the displacement detecting means is not within the specified range. Such a range may be determined such that the detector can calculate the distance with an error small enough to be practical as long as the displacement in the direction of the emitted light is within this specified range. The warning means may output a warning signal by displaying a warning indicator or a vocal message, reminding the driver of the vehicle that the detector is not operating normally. With the detector thus prepared, accidents caused by a failure of the detector to measure the distance to an object correctly can be prevented.
The detector of the invention is intended to be installed on a motor vehicle and may also comprise speed detecting means such as a speedometer for detecting a running speed of the motor vehicle on which it is installed, and speed judging means for judging whether or not the motor vehicle is traveling at a stable rate on the basis of outputs from the speed detecting means such that the displacement detecting means detects the directional displacement only when the speed judging means judges that the motor vehicle is running at a stable rate. If such a control unit is provided, miscalculations of the displacement of the direction of light emission can be prevented because the displacement is calculated only when the motor vehicle is running smoothly at a constant speed.
A method according to this invention of detecting an object on a road surface may be characterized as comprising the steps of emitting a beam, receiving a reflected beam of the emitted beam by an object, measuring a distance to the object based on the received reflected beam, recognizing an overhead beam reflector with a known height based on the received reflected beam, identifying a distance to the overhead beam reflector immediately before the overhead beam reflector becomes unrecognizable, and detecting a directional displacement of the emitted beam based on the distance to the identified overhead beam reflector. The method may further comprise a step of correcting the directional displacement.
In summary, with the detector and detection method embodying this invention as described above, a beam is emitted and when it is reflected by an object on a road surface and the reflected beam is received, the distance to the reflecting object is calculated on the basis of the reflected beam which has been received. When an overhead beam reflector with a known height is recognized based on the received reflected beam, the distance to such an overhead beam reflector is calculated and its value immediately before it becomes unrecognizable is identified. The displacement of the direction of the emitted beam is detected based on this identified distance and may be corrected.
A detector according to a second embodiment of the invention may be characterized as being similar to the one according to the first embodiment described above but different therefrom wherein the detector is provided with judging means for judging whether or not the object which reflected the emitted beam and received by the receiving means was the road surface itself and distance identifying means for identifying a distance to the road surface where the reflection took place if the judging means judges that the object which reflected the emitted beam and received by the receiving means was the road surface, and the displacement detecting means detects the displacement based on the distance to the beam-reflecting road surface identified by the distance identifying means. The detector may further comprise correcting means for correcting the detected displacement based on the distance to the road surface.
A method according to the second embodiment of the invention may be likewise characterized as being similar to the one according to the first embodiment described above but different therefrom for comprising the steps of judging whether or not the object which reflected the received beam was the road surface and identifying a distance to that light-reflecting part of the road surface if it has been judged that the beam-reflecting object was the road surface itself.
The first and second embodiments may be combined such that the angle of emitted beam can be detected and corrected, if necessary, by receiving a reflected beam and recognizing this either as a beam reflected from an overhead beam reflector of a known height or a beam reflected by a road surface.
As a further variation, the angle of emitted beam may be detected and corrected by calculating the slope of the road surface from where the reflected beam is received.